Light is both a photographer's best friend and worst enemy. Consider what recently happened to me, for instance. I visited a wild animal park with the hope of taking some pictures of wolves, cougars, and bears. Unfortunately, it had rained all morning, and the sky was dark and overcast when I arrived at noon. As I prepared to take my first photo, I found that the camera was so starved for light that it wanted to set the exposure at about 1/10 second--far too slow to get a sharp image of active animals, especially since I was hand-holding the camera.
So what did I do? I increased the ISO. This week, let's discuss ISO and how you can use it to take pictures in situations when normal camera settings just won't do.
What Is ISO?
As you might recall, 35mm camera film comes in a variety of speeds. Film speed is measured using a numbering system called ISO, for the International Standards Organization; it's sometimes also referred to by its older name, ASA, for the American Standards Organization. Films with lower ISO numbers are known as slow, or less sensitive to light; films with higher ISO numbers are faster, or more light-sensitive.
When using a film camera, it's pretty typical to shoot with ISO 100 or 200 film in normal daylight, and use ISO 400 film for lower-light photography. Super-fast films like ISO 800 and even ISO 1600 are available for photography in near darkness, but they're rarely used because of their inherent limitations.
Here's the secret that governs film speed: Doubling the ISO number of the film doubles its sensitivity to light. So ISO 200 film needs half the light to take the same picture as ISO 100 film. ISO 400 film needs a quarter of the light that ISO 100 needs. In other words, you could capture a low-light scene with a shutter speed of 1/15 second with ISO 100 film, or 1/60 second with ISO 400. That's an incredibly powerful capability that means the difference between getting a blurry mess and a sharp photo.
What ISO Means for Digital Photography
Big deal, right? We're dealing with the digital world: We don't buy memory cards with ISO numbers marked on them. What does all this have to do with us?
As you probably know, you can control the sensitivity of your digital camera's light sensor in a way that mimics the experience of loading different-speed films into your 35mm camera.
By default, most digital cameras use an ISO rating somewhere between 64 and 200, with the most common default being about ISO 100. Getting more sensitivity out of your digital camera is as simple as selecting a higher ISO from the camera's menu. Most digital cameras offer a range of ISO values, such as 100, 200, and 400. Some go much higher--all the way to 3200. Just dial in the ISO you need.
The Dark Side of ISO
That sounds great, right? So if higher-ISO film is more light-sensitive, why bother using low-ISO film at all? And in digital photography, why not always use a high ISO number? Unfortunately, light sensitivity brings its own baggage. High-speed film is notoriously grainy. As you increase film's sensitivity to light, the light-sensitive grains of silver halide in the film emulsion get bigger and more noticeable. In the final print, that manifests itself as grain--irregular elements that mar the photo's smooth, high-resolution look. That's why 1600-speed film is so rare; the grain on that film is monstrously large, making the overall image look like something shot by a bank security camera.
In digital photography, we get almost exactly the same effect with higher ISO values. Increasing the sensitivity of the camera's light sensor introduces noise into the photo--random pixels of colour.
You may not notice noise from a distance, but if you zoom in on a digital photo on a display or look closely at a print, it's hard to miss. All digital images have some amount of noise, and it gets worse as the ISO goes up. Take a look at this picture. You may not notice the noise in a casual viewing, but check out this detail I've zoomed in on; the noise is quite apparent, appearing as colored speckles that are most evident in the wall at the back of the scene.
So what should you do about ISO? I suggest shooting at the lowest possible ISO all the time.
In ordinary conditions, stick with the camera's lowest ISO level, since that'll give you the least digital noise. But when you notice that the camera is recommending a really slow shutter speed (less than about 1/30 second for handheld shots with a point-and-shoot), crank up the ISO. Just remember to drop it back down to the lower value when you're done, so you don't accidentally capture a month's worth of pictures at ISO 800.
Another thing to keep in mind: Most digital cameras don't allow you to adjust the ISO, or any other setting for that matter, when you're in Automatic Exposure mode. To tweak the ISO, you'll want to be in Program, Shutter Priority, Aperture Priority, or a Scene mode.